In order to monitor information inside a tire, such as an air pressure and a temperature, there has been heretofore performed attachment of a sensor unit to an inner surface of the tire, a well part of a rim or the like, detection of the information inside the tire by use of the sensor unit, and transmission of the detection results to a receiver outside the tire by utilizing radio waves (for example, see Japanese patent application Kohyo publication No. Hei 10(1998)-504783).
In such a sensor system for a tire, a sensor unit is installed in a tire's air chamber. Thus, there is a drawback that, even if the sensor unit falls off a predetermined installation position, the falling of the sensor unit cannot be known. If a vehicle runs while the sensor unit has fallen off, an electronic component of the sensor unit will be destroyed, and communication may be disabled or a false alarm may be given.